In recent years, an aqueous dispersion has attracted attention as a material for the shift from an organic solvent system to a waterborne system in the coating field. Waterborne paints obtained from a aqueous polymer dispersion, however, has not yet exhibited sufficient physical properties in terms of pigment dispersibility, water resistance, antifauling, hardness and the like, as compared with organic solventbased paints. The aqueous polymer dispersion here refers to a liquid form in which a polymer is dissolved and/or dispersed in water.
For the purpose of enhancing the function and physical properties of a polymer in water, or for the purpose of enhancing the function of an inorganic solid particle in water, an aqueous dispersion of composite particles in which a polymer and an inorganic solid particle are compounded has been developed.
As the method for producing the aqueous dispersion of composite particles, for example, proposed are an emulsion polymerization method in which a radical polymerizable group is functionalized into a silica particle, a method in which an inorganic solid particle and a polymer are subjected to composite formation by means of a nonionic surfactant having a low clouding point, and an emulsion polymerization method in which the amount of a polymerizable monomer to be fed into a reaction mixture is controlled and composite formation is conducted (Patent Literatures 1 to 4). A method in which a radical polymerizable group is functionalized into a silica particle, the CMC (critical micelle concentration) of an anionic surfactant is determined with respect to the silica particle, the pH is adjusted to 7.5, and a monomer is subjected to emulsion polymerization in a reaction mixture including the silica particle and the anionic surfactant in an amount of CMC or less, however, has the problem of instability during emulsion polymerization and an easy occurrence of an aggregate. In addition, a method in which a radical polymerizable group is not functionalized into a silica particle, the pH is adjusted to 7.5, and a reaction mixture including the silica particle and the anionic surfactant in an amount of CMC or less is used cannot allow a water dispersion to be stably produced (Patent Literature 1).
In the method in which an inorganic solid particle and a polymer is subjected to composite formation by means of a nonionic surfactant having a low clouding point, the inorganic solid particle can be encapsulated every one particle, but it is difficult to stably store a water dispersion for a long period (Patent Literature 2).
In the emulsion polymerization method in which the amount of a monomer to be first fed into a reaction mixture is controlled and composite formation is thus conducted, the polymerization system is adjusted to be neutral or alkaline, and a nonionic surfactant having an ethylene oxide unit, or an alkylarylsulfonic acid is used as a surfactant, but the resulting dispersion is aggregated or polymerization hardly progresses on the surface of an inorganic material, and therefore a particle of a polymer itself is generated separately from an inorganic solid particle. Therefore, as disclosed, a single particle including a plurality of inorganic particles, referred to as a raspberry type, is obtained by emulsion polymerization in which various inorganic solid particles, and a functional monomer and an initiator that are oppositely ionic thereto are combined (Patent Literatures 3 and 4).
Instead of the method in which the amount of a monomer to be first fed into a reaction mixture is controlled, disclosed is a method in which emulsion polymerization is conducted by feeding a monomer into a mixture of an anionic surfactant and water in the presence of a mixture obtained by mixing of a polymer having a sulfonic acid group or a sulfoalkyl group, produced by solution polymerization, and an inorganic solid particle (Patent Literatures 5, 6).
A method is disclosed in which a silane coupling agent having a radical polymerizable group is used to functionalize an inorganic solid particle surface with the radical polymerizable group, and a monomer having a hydroxyl group is used in combination therewith to thereby subject the inorganic solid particle and a polymer to composite formation. But a single particle including a plurality of the inorganic particles is formed or an aggregate is generated in a large amount, while the inorganic particle is encapsulated with a part of the polymer, (Patent Literature 7).
As a method for reducing the zeta potential of an aqueous polymer dispersion, disclosed is a method in which a monomer having an anion group, such as itaconic acid and phosphoric acid ester monomers, is subjected to emulsion polymerization (Patent Literatures 8 and 9).
Furthermore, disclosed is a method in which an inorganic solid particle having a high surface charge density and a polymer are subjected to composite formation (Patent Literature 10). The surface charge density, in which, while the analysis method thereof is not specifically disclosed, the numerical value thereof is 2 to 10 μC/cm2, is thus presumed to correspond to σa (analytical surface charge density) described in a common literature (for example, Non Patent Literature 1). In Examples of Patent Literature 10, there is the following description: a dispersion is subjected to dialysis and ion exchange treatments. While no specific description other than this is disclosed in Patent Literature 10, the surface charge density is measured as the numerical value quantitatively determined by electrical conductometric titration after the above treatments and the acid group on the particle surface is merely subjected to quantitative determination. In general, the zeta potential of a particle in a dispersion means the electrical mobility measured in the presence of various impurity ions in the dispersion without any dialysis and ion exchange, and corresponds to the numerical value representing the surface state of the particle under a condition where the dispersion is actually used. Even if electrical conductometric titration is conducted in the same condition, the numerical value determined is in consideration of various ion substances, and even the surface charge density cannot be gained. The composite particle disclosed in Patent Literature 10 achieves a high surface charge density by use of a large amount of an ionic monomer, thereby resulting in production of a particle of a single polymer, in addition to the inorganic solid particle, to cause the problem about warm water resistance of the coating film thereof. Furthermore, as disclosed in Examples, a method in which dispersion polymerization is conducted in the presence of a silica particle in an alcohol is proposed, but the particle cannot be dispersed in water.